Elder, eldest or older, oldest?

Elder and eldest mean the same as older and oldest. We only use the adjectives elder and eldest before a noun (as attributive adjectives), and usually when talking about relationships within a family:

• Let me introduce Siga. She’s my elder sister. Not: … She’s my sister. She’s elder.
• It’s Catherine’s eightieth birthday on Thursday and she is now the eldest member of the family.

We also use elder and eldest as nouns:

• Jack is the eldest of four brothers.

Older and oldest are used in similar ways:

• My older sister is coming to stay with us at the weekend.
• Matt is the oldest of our children and Simon is the youngest.

Older and oldest can be used to refer to the age of things more generally:

• The town hall is by far the oldest building in the whole region.
• Not: The town hall is by far the eldest building …

We can use older and oldest after a linking verb (as predicative adjectives):

• I think her grandfather must be older than her grandmother.
• Not: I think her grandfather must be elder than …